At the end of larval development in Drosophila melanogaster a rise in the titer of ecdysone directly stimulates the expression of a small number of genes (early genes). The protein products of these genes are responsible for the subsequent stimulation of a much larger number of genes (late genes) whose expression is necessary for normal metamorphosis. Three of these early genes, 2B5, 74EF, and 75B, have been analyzed in some detail. The identity of specific late genes and their regulatory relationships to specific early genes are poorly known. These relationships determine the coordinate and tissue specific expression of the late genes. Two enzymes (arginine kinase and pyruvate kinase) in D. melanogaster have been shown to be hormonally regulated during the prepupal period of development and this developmental regulation is influenced by the 75B early gene. To determine the molecular mechanisms responsible for this coordinate regulation requires that the genes responsible for these enzymes be cloned and characterized. The gene for arginine kinase has been isolated under other support. This proposal outlines the isolation and initial characterization of the gene from Drosophila melanogaster for pyruvate kinase.